Fe-based amorphous soft magnetic material is generally used as high-frequency soft magnetic materials have high saturation magnetic flux density (Bs) but exhibit low magnetic permeability, large magnetostriction and poor high-frequency characteristic, and Co-based amorphous soft magnetic materials have demerits such as low saturation magnetic flux density and high price due to their limit as a raw material. Further, amorphous soft magnetic alloys are difficult to process in the form of strips and give a limit to the shape of the products like toroidal shape, and ferrite soft magnetic materials exhibit low loss at high frequencies but are hard to process to a small size because of their low saturation magnetic flux density. Both the amorphous soft magnetic material and the ferrite soft magnetic material display poor reliability in the aspect of thermal stability due to their low crystallization temperature.
Currently, the soft magnetic core is obtained by winding an amorphous ribbon prepared through a RSP technique in the form of a core. In this case, the soft magnetic core exhibits considerably poor levels of DC bias characteristic and high-frequency magnetic permeability and has a relatively large core loss. The reason lies in the fact that the amorphous ribbon core has no air gap in the ribbon, while the powder core includes air gaps uniformly distributed between the powders. It is therefore preferable to use the powder core having air gaps uniformly distributed in order to obtain a core with excellent high-frequency magnetic permeability and core loss characteristic.
The soft magnetic core used in the choke coils for the purpose of reducing or smoothing electromagnetic noise is manufactured by coating a ceramic insulating material with a metallic magnetic powder, such as of pure iron, Fe—Si—Al alloy (hereinafter, referred to as “sandust”), Ni—Fe—Mo permalloy (hereinafter, referred to as “MPP (Moly Permally Powder)”), Ni—Fe permalloy (hereinafter, referred to as “high flux”), Fe-based amorphous powder core, nano-crystalline powder core, etc., adding a mold lubricant and then performing the subsequent steps of pressurization, molding and heat treatment.
In the conventional manufacturing method of a soft magnetic core, an insulating layer is formed between the powders to uniformly distribute air gaps, which can minimize the eddy current loss that abruptly increases at high frequencies and maintain the air gaps entirely to secure excellent high-current DC bias characteristic. For example, the pure iron powder core is used to reduce the electromagnetic noise that is biased by the high-frequency current in the choke coil of a switching mode power supply (SMPS) having a switching frequency of 50 KHz or below, and the sandust core is used as a core for the secondary smoothing choke coil of a SMPS having a switching frequency in the range of 100 KHz to 1 MHz and a core for noise reduction. In this regard, the term “DC bias characteristic” as used herein refers to the characteristic of a magnetic core with respect to the waveform in which the direct current (DC) is biased by a weak alternative current (AC) generated when the AC input of the power supply is converted to the DC. If the DC is biased by the AC, the magnetic permeability of the core drops in proportion to the DC. In this regard, the DC bias characteristic is evaluated in terms of the percentage of permeability (% μ; percent permeability) with the DC bias in relation to the permeability without the DC bias.
The MPP core and the high flux core, which are used in the same frequency range of the sandust core, may have more excellent DC bias characteristic and lower core loss than the sandust core but are expensive. Accordingly, there still remains a need to develop a core that is of low price but with equivalent levels of characteristics to the MPP or high flux core. In the meanwhile, it is more difficult to meet the requirements to the soft magnetic cores for such use purposes in terms of the characteristics associated with the recent trend towards the miniaturization, higher integration and higher reliability of the SMPS.
The core for smoothing choke coils in the SMPS is required to have appropriate inductance L, low core loss and excellent DC bias characteristic. To meet these requirements, Korean Patent No. 10-0545849 suggests a manufacturing method of an amorphous soft magnetic core using a composite powder comprising Fe-based amorphous powder of which the particle size distribution is controlled such that 34 to 45% of the powder passesthrough a 100 to +140 mesh sieve (107 to 140 μm) and 55 to 65% of the powder passes through a −140 to +200 mesh sieve (74 to 107 μm).
But, the particle size distribution adopted in the above patent composes the powder having a large particle size greater than 100 μm to be a great proportion in the composite powder, so there appear excessively large-sized pores between the powders. Particularly, in the case of amorphous powder, such a large pore size is not substantially reduced during the subsequent molding process in consideration of the fact that the amorphous powder is scarcely susceptible to plastic deformation under the molding pressure during the molding process. This can give a limit as to enhancing the DC bias characteristic. Further, an excess of pores between the powders results in the lower strength of the molded products, adversely affecting the handleability or workability of the products. Also, another problem lies in that an increase in the particle size of the powder increases the eddy current loss, which entirely makes it difficult to reduce the core loss to less than 1,000 mW/cm3 (Refer to Table 1 in Korean Patent No. 10-0545849).
It is undesirable for the fine powder having an extremely small particle size to take a relatively large proportion and increase the hysteresis loss. Generally, the core loss can be divided into hysteresis loss and eddy current loss. The hysteresis loss represents a loss as much as the area of a magnetic hysteresis loop. The eddy current loss indicates a power loss as a result of the eddy current generated by the induced electromotive force. Such an eddy current loss can be represented by the following expression, which shows that the eddy current loss is proportional to the square of the particle thickness (diameter) in the core:
                    P        e            ⁡              (                  eddy          ⁢                                          ⁢          current          ⁢                                          ⁢          loss                )              =                            π          2                ⁢                  B          2                ⁢                  f          2                ⁢                  d          2                            C        ⁢                                  ⁢        ρ                        B      =              Flux        ⁢                                  ⁢        Density              ,          f      =      Frequency        ,          d      =      thickness      Accordingly, reducing the particle size of the powder may entirely reduce the eddy current loss, but it can also reduce the magnetic permeability and increase the Hc of the magnetic hysteresis loop, leading to an increase in the hysteresis loss. It is therefore necessary to limit the content of the fine powder having a particle size less than 50 μm.
Moreover, Server PCs, Telecom Power, or the like are currently leading the industries of the switching mode power supply (SMPS) and the major SMPS makers are IBM, DELL, HP, etc. With the trends of PCs towards the higher capacity, higher quality and slimness, there has been a step change in the design specification of the power supply. First of all, the specifications of the CPU have a tendency to higher frequency and higher current, and therefore, the stable power supply has become a big issue. In addition, the capacity of the power supply has increased in accord to the trends of the PCs to the multi-functionality. Accordingly, the addition of a power-factor improvement circuit is now mandatory, and a power core with great current stability, frequency stability and low loss is demanded as a choke for high-performance PFC in order to minimize the increase in the volume of the power supply as a result of the addition of the power-factor improvement circuit. For the sake of meeting the practical demands, it is necessary to improve the magnetic characteristic of the soft magnetic core using the Fe-based amorphous powder suggested in Korean Patent No. 10-0545849 to the level as required in the market.
As a result of the mentioned studies on the manufacturing method of a Fe-based amorphous soft magnetic core on the above-described background, the inventors of the present invention have found out that the particle size distribution of the powder constituting the soft magnetic core can be efficiently controlled to the optimum to increase the molding density of themolded core material, enhance the high-current DC bias characteristic and improve the core loss characteristic, thereby completing the present invention.